Method for detecting collisions on and controlling access to a transmission channel

ABSTRACT

A method for controlling multiple access of a transmission channel wherein a plurality of different patterns are assigned to a plurality of sending stations so that each sending station corresponds to a unique pattern, preferably a pattern represented by a Binomial coefficient. Each unique pattern is transmitted from a corresponding sending station to the transmission channel by way of a control minislot. Ternary feedback is received from the control minislot. A summation of different patterns within each control minislot are analyzed to detect whether a collision exists between the different patterns within each control minislot.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for detecting collisions in atransmission channel using a distributed queueing random access protocol(DQRAP) wherein broadcast channel time is divided into a plurality ofslots, each of which includes one data slot and one or more controlminislots, and each of a plurality of sending stations maintains twocommon distributed queues. One queue, the data transmission queue, isused to organize the order of data transmission, and the other queue,the collision resolution queue, is used to resolve collisions that haveoccurred and to prevent collisions by new arrivals. The protocolincludes data transmission rules, request transmission rules andqueueing discipline rules.

2. Description of Prior Art

Investigation of multiple, random access control methods has been anactive research area since as early as 1970. The well known CSMAprotocols were then developed and later followed by multiple accessmethods which utilized various forms of feedback to improve performanceby reducing or avoiding the occurrence of collisions. These includedcollision resolution schemes, now called tree-and-window collisionresolution algorithms (CRA). The CSMA protocols achieved high throughputwith minimal delay with low offered loads, and they have gained wideapplication in local area networks. In fact with zero propagation delay,collisions in slotted CSMA can be completely avoided and the performanceof CSMA then corresponds to that of a perfect scheduling protocol, suchas an M/D/1 queue. However, the CSMA protocols are not stable whentraffic is heavy and while dynamic control mechanisms can improveperformance, the unstable nature cannot be changed.

The first CRA included a tree algorithm which achieved a maximumthroughput of 0.43, and was stable for all input rates of less than0.43. This stable characteristic of the tree algorithm has attractedmuch attention in both the communications and information theory areas.The tree algorithm was improved by increasing the maximum throughput to0.462. The next improvement was the 0.487 window protocol. The tree andwindow protocols are based on efficient use of channel feedback toresolve collisions and require transmitter coordination. It has beenshown that the upper bound of throughput of all algorithms based onternary feedback is 0.568, the tightest upper bound to date.

It is widely believed that the best achievable throughput is in theneighborhood of 0.5. If the amount of channel feedback is increased toindicate the number of packets involved in each collision, thenthroughput up to one may be achieved. However, the known algorithms inthis context achieve only 0.533 throughput. Some known protocols achievehigher throughput than 0.5 by using control minislots (CMS) to obtainextra feedback. Among such known protocols, the announced arrival randomaccess protocols (AARA) achieve the best performance with respect tothroughput and delay characteristics. With three minislots the AARAprotocol achieves a throughput of 0.853. However, to achieve throughputapproaching one, the AARA protocol must use an infinite number ofminislots. Obviously, the AARA protocols do not achieve or approach thebound of performance in this context. All existing tree protocols seemto use data slots to resolve collisions. In this process, channelcapacity is lost either to empty slots or to collisions. All suggestedimprovements to tree protocols increased the channel throughput byreducing empty slots and collided slots, but none eliminated this typeof loss.

SUMMARY OF THE INVENTION

It is one object of this invention to provide a method for controllingmultiple access of a transmission channel through the detection ofcollisions by comparing a plurality of different patterns within acontrol minislot.

It is another object of this invention to assign different Binomialcoefficients to a plurality of sending stations and to use such Binomialcoefficients to detect collisions.

It is still another object of this invention to use a distributedqueueing random access protocol in various systems, such as with packetradio, satellite, broadband cable, cellular voice, and other passiveoptical networks.

The distributed queueing random access protocol (DQRAP) of thisinvention is a stable random multiple access protocol for use in abroadcast channel shared by an infinite number of bursty stations. TheDQRAP according to this invention is based on tree protocols withminislots. These tree protocols use minislots to provide extra feedbackin order to reduce the number of empty and collided slots. However, theDQRAP according to this invention uses the minislots for collisionresolution and resolves the data slots for data transmission.Implicitly, even though counters are often used, conventional treealgorithms use a single queue which performs as a collision resolutionqueue. The method according to this invention achieves the desiredperformance by introducing an additional queue, the data transmissionqueue, to schedule data transmission parallel to contention resolutionand thereby nearly eliminating contention in the data slots. The DQRAPof this invention, using as few as three minislots, achieves aperformance level which approaches that of a hypothetical perfectscheduling protocol, such as the M/D/1 system, with respect tothroughput and delay.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and objects of this inventionwill be better understood from the following detailed description takenin conjunction with the drawings wherein:

FIG. 1 is a diagrammatic representation of a slot format according toone preferred embodiment of this invention, which includes a data slotand a variable number of control minislots;

FIGS. 2a and 2b are diagrammatic representations of sequenced events foran enable transmission interval (ETI) and a contention resolutioninterval (CRI);

FIG. 3 is a diagrammatic view showing the operation of one preferredembodiment of a distributed queueing random access protocol (DQRAP)according to this invention;

FIG. 4a shows a diagrammatic view of sending stations transmittingcontrol minislots and a data slot during one slot time, according to onepreferred embodiment of this invention;

FIG. 4b shows a diagrammatic view of the control minislots and the dataslot being fed back to each sending station, in response to theinformation transmitted, as shown in FIG. 4a;

FIG. 4c is a schematic diagram showing a DQRAP according to onepreferred embodiment of this invention, which is modeled as a queueingsystem having two subsystems, a queueing contention resolution subsystem(QCR) and a data transmission subsystem (DT);

FIG. 5 is a table showing values of CRI lengths (L_(n)) as a function ofdifferent values of n;

FIG. 6 is a table showing maximum input rates and the correspondingwindow sizes as a function of the minislot number;

FIG. 7 is a graph showing the actual throughput of the DQRAP of thisinvention as a function of the input rate and the number of minislots,with the overhead equal to 0.01;

FIG. 8 is a graph showing a relationship of the maximum actualthroughput as a function of one minislot overhead and the minislotnumber;

FIG. 9 is a graph showing the percentage of the first access throughputof the DQRAP of this invention, as a function of the input rate;

FIG. 10 is a table showing the average delay and deviation between theDQRAP according to this invention and an M/D/1 system, wherein thenumber of minislots is equal to three;

FIG. 11 is a table showing a simulated average delay and thecorresponding deviation of the DQRAP of this invention, with a varyingnumber of minislots; and

FIG. 12 is a graph showing simulated results of the average delay as afunction of the input rate, for the DQRAP of this invention with threeand sixteen minislots, as compared to that of an M/D/1 system.

DESCRIPTION OF PREFERRED EMBODIMENTS

Conventional multiple access protocols have proposed the use of controlminislots (CMS), in addition to a dataslot (DS), to provide binary orternary feedback in order to improve performance. Most conventionalmultiple access protocols have never been implemented, possibly becausethe improvement in performance has not been sufficient to offset theoverhead of the CMS. However, Distributed Queueing Random AccessProtocol (DQRAP) according to this invention provides a performancewhich approaches that of a hypothetical perfect scheduling protocol,such as the M/D/1 system, with respect to throughput and delay. TheDQRAP of this invention achieves this performance using as few as threeCMS providing ternary feedback. The method of this invention can beachieved with several preferred embodiments for acquiring ternaryfeedback in a variety of media.

According to this invention, ternary feedback is defined as the abilityof a receiver to differentiate between three conditions: (1) no signalpresent or an absence of a pattern; (2) a single signal present or apresence of only one pattern; and (3) multiple signals present or apresence of two or more patterns. These events occur when sendingstations 24 operate under a protocol which permits sending stations 24to transmit either arbitrarily or under rules which may allow two ormore sending stations 24 to simultaneously transmit on the sametransmission channel 30. FIG. 1 shows one preferred layout of DS 32 andCMS 34. The length of one CMS 34 is preferably minimized according tothis invention, because if each CMS 34 requires one-third, for example,of the overall slot in a system using three CMS 34, then the entirebandwidth would be consumed by the CMS 34. According to one preferredembodiment of this invention, each CMS 34 uses a minimum of bandwidthand a receiver is able to discriminate between no transmission, onetransmission, and more than one transmission in one CMS 34. Ideally,each CMS 34 should utilize something less than 1% of the slot so thatoverhead for a three-minislot system is well under 10%. If the viabilityof acquiring ternary feedback can be demonstrated, then the onlyobstacles to implementing the DQRAP according to this invention arethose common to other conventional communications protocol.

According to one preferred embodiment of this invention, an energy levelthreshold ternary feedback (ELTTFB) is implemented by having eachsending station 24 transmit a signal for the duration of one CMS at apower level such that the signals from sending stations 24, after normaltransmission loss, each arrive at the receiver with the same power. Theadditive nature of the energy in the arriving signals indicates that areceived signal level above a given threshold can be treated as acollision. This is feasible in a fiber optic environment wheninformation is transmitted by turning the light source on and off. In afiber optic system, it appears feasible to assign a CMS duration ofabout three or four bit times. This implies an overhead of about 12 bitsadded to a sample asynchronous transfer mode (ATM) cell, having 424bits, for overhead of 2.75%, which is well under a 10% objective forthis invention.

According to another preferred embodiment of this invention,combinatoric ternary feedback (CTFB) can be used, particularly in anenvironment where it may be difficult to implement the previouslydescribed ELTTFB method. CTFB is preferably implemented by assigningeach station a value C(n,k) which represents one of a plurality ofpatterns when k items at a time are selected from n objects, whichcorresponds to the Binomial Theorem. For instance, if there are 10stations then a coefficient of C(5,2) provides 10 different patterns,each with two `1s`. According to such coefficient C(5,2), patterns couldbe assigned to each sending station 24 wherein:

Station 1 transmits 11000;

Station 2 transmits 10100;

Station 3 transmits 10010;

Station 4 transmits 10001;

Station 5 transmits 01100;

Station 6 transmits 01010;

Station 7 transmits 01001;

Station 8 transmits 00110;

Station 9 transmits 00101; and

Station 10 transmits 00011.

If the receiver detects more than two `1s` arriving in one CMS 34period, then a collision within transmission channel 30 has occurred.Such detection method is simplified according to this invention sincethe receiver is looking only for the presence of a signal rather thanattempting to assess whether a received signal is over or under a giventhreshold. The number of `1s` can be determined by counting pulses or byintegrating over a time period of one CMS 34 and comparing with athreshold.

When using the CTFB method, the overhead is greater than when using theELTTFB method but the CTFB method of this invention is still practicalfor many applications. For instance, in an ATMLAN with 64 stations, acoefficient of C(8,4) provides 70 different available patterns. Overheadfor three CMS in an ATM Cell 24/424 is 5.67%, again well under anarbitrary overhead limit of 10%. Collisions are easier to detect withthe CTFB method than with the ELTTFB method. One advantage of the CTFBmethod is that sending station 24 can be identified since each sendingstation 24 is transmitting a unique pattern.

In another preferred embodiment according to this invention, a digitallogic ternary feedback (DLTFB) method is used to detect collisions. Intrue bus systems, signals arriving from two or more sources arephysically or'ed and the signal levels are essentially summed. Thisaspect is assumed in the previously described ELTFB and CTFB methods. Indigital systems such as 56 kbps DDS, T1, etc., separate signals are notphysically combined since this would produce undefined results. Instead,logic either is used to produce a new signal depending upon the inputsor is used to gate one of the input signals to the output. The DLTFBmethod according to this invention is easily implemented in thesesystems.

The operation of the DLTFB method can be defined in practical termsusing telephone terminology. Assume that a WAN network of 56 kbps leasedlines is designed as a multi-drop network. In each city on the networkthere is a junction point at the point of presence (POP) of theinterexchange carrier. The sending station in each city is connected tothe interexchange circuit (IXC) at the POP via a local channel providedby the local carrier. When a sending station opts to write in aparticular CMS a 00 or a 01 is transmitted, 00 representing notransmission and 01 representing a transmission. The timing, provided bythe outbound circuit, is such that the two bits arrive at the POPjunction at exactly the same time as the CMS arriving on the inbound IXCfrom another city. The logic at the junction compares the two inputs andtransmits according to the following table:

    ______________________________________                                        IN(1)          IN(2)                                                          (IXC)          (LOCAL)   OUT                                                  ______________________________________                                        00"E"          00"E"     00"E"                                                00"E"          01"S"     01"S"                                                01"S"          00"E"     01"E"                                                01"S"          01"S"     11"C"                                                11"C"          00"E"     11"C"                                                11"C"          01"S"     11"C"                                                ______________________________________                                    

The letters "E", "S", and "C" represent empty, successful and collision,respectively, the terms usually used in the protocols which employternary feedback. There are three possible inputs from the inbound IXCsince there may have been a collision at a previous city on the "tree".

Three CMS require 6 bits and thus the overhead on the previouslydescribed ATM cell is 1.42%. The efficiency of the DLTFB methodaccording to this invention permits the CMS size to be increased so asto include requests for a specific number of frames, and to includepriority levels.

Where synchronization is available, the DLTFB method according to thisinvention is preferred. However, if synchronization on an approximately56 Kbps or higher digital circuit cannot be guaranteed such that asequence of two bits representing the transmission in one CMS 34 by onesending station 24 does not arrive at a common point at the same time asthe two bits representing the transmission of another sending station24, then the CTFB method according to this invention should preferablybe used. The number of sending stations 24 will normally determine thesize of the coefficient C(n, k) to which a number of guard bits areadded to compensate for the lack of synchronization. When two CMS 34mapped in such fashion arrive at common point, the fact that one of theslots may be one or more bits out of synchronization with the other willnot matter since the resulting transmission will represent an illegalpattern or a pattern which indicates a collision. The receiver then needonly search in a range including the guard band for an acceptablepattern containing all zeros, or some other suitable pattern which wouldindicate a collision.

According to still another preferred embodiment of this invention, thecarrier combinatoric ternary feedback (CCTFB) method is used to detectcollisions. The CCTFB method is applicable to broadband systems whichuse a modulated carrier to convey digital data. Such broadband systemsinclude CATV systems, packet radio systems, cellular radio systems,satellite systems, and fiber optic systems where a modulated carrier isused. The normal lock-on and synchronization to data of such systemsintroduce very high overhead values if normal data transmission is usedin CMS. For instance, in a typical 9600 bps packet radio systemutilizing a 450 MHz carrier the typical data lock-on and synchronizationperiod is 10 ms-20 ms. Three such periods could sum to as much as 60milliseconds, longer than the previously discussed ATM cell which at9600 bps is 44 milliseconds. Aside from the overhead there is a problemwith the "capture" effect prevalent in many conventional systems whichutilize modulated carriers. In such a system a receiver locks onto asingle received carrier, disregarding other carriers. This is adesirable trait in normal circumstances and is designed into most radiosystems. But if the object is to detect the presence of two or moresignals, the capture effect is disastrous.

The CCTFB method according to this invention uses the capture effect toidentify the presence of two or more signals. Each station is assigned acoefficient C(n,k) pattern as previously described. Each CMS is thenallocated a duration sufficient to contain n individual signals oflength t. Each of the n signals are a burst of carrier of length t Ifthe C(n,k) pattern has adjacent `1s` then the carrier just remainsturned on for each of the adjacent `1s`. Simply put, sending station 24transmits in one CMS 34 by turning the carrier on or off according tothe C(n,k) pattern. Each carrier on a period corresponding to a "1" inthe pattern is of length t seconds. The receiver 26 uses conventionalfiltering and phase locked loop (PLL), or another suitable detectiontechnique, to detect the carrier. For a C(n,k) system, a CMS has a basictime period of nt seconds. A transmitting station turns on its carrierfor k periods of t seconds during one CMS 34 interval. The receiverlocks onto the arriving carrier and either by counting cycles orintegrating over the CMS period makes an estimate of empty representedby no signal present in one CMS 34, successful represented by only onesignal present in one CMS 34, or collision represented by two or moresignals present in one CMS 34. The carrier will be present for notransmission, present for kt seconds for a single transmission, andpresent for more than kt seconds for a collision.

There are two major advantages to using the CCTFB method of thisinvention in those systems where it is feasible. First, the period t canbe less than one or two hundred cycles of the carrier. This is more thanadequate for the receiver 26 to detect and lock on. Most carriers ofinterest operate at frequencies higher than 20 MHz. A period t of 200cycles at 20 MHz takes 10 microseconds. Even in a C(n,k) system wheren=16, the CMS 34 time period would be: 16×10 microseconds (μs)=0.16milliseconds (ms), for a total of 0.48 ms. This means that slots can beas short as 10 ms and an overhead constraint of less than 5% would besatisfied. As the carrier frequency increases, the data carryingcapacity is increased. However, frame sizes usually remain at the samesize making the duration of the data slots shorter. One advantage of theCCTFB method of this invention is that as the carrier frequencyincreases, the time duration of the CMS, at 200 cycles, is reduced sothat the overhead remains proportionally the same. In fact, at speedsenvisioned in fiber optic networks the CMS overhead could becomeminuscule. Maximum on-off repetition rates can be set to ensure thatbandwidth limits are not exceeded.

Second, the capture effect is used to an advantage. If two or moresending stations 24 transmit in the same period t(i) in one CMS 34, thecapture of one signal contributes to the total count in the CMS period.The total count will then exceed the expected count of k. If there isinterference and neither signal is received, then the decision about acollision must be determined from the remainder of the signal. However,with a C(8,4), the minimum practical value of n and k in one preferredembodiment of this invention, the probability that two or more collidingpatterns in one CMS 34 will result in a count of 4 is relatively low,even if the capture effect fails.

The preferred methods of this invention are particularly suitable foruse with a communication system accommodating or serving an infinitenumber of sending stations 24 or bursty stations which communicate overa multiaccess and noiseless broadcast channel. Sending stations 24preferably generate single messages of fixed length. Transmissionchannel 30 is preferably divided into slots of fixed length. As shown inFIG. 1, each slot comprises a variable number m of CMS 34 followed by asingle DS 32. The size of one DS 32 is assumed to be of length 1, equalto the length of messages generated by each sending station 24. Each CMS34 is assumed to be of a length δ. The size of δ is implementationdependent but δ is assumed to be much smaller than the corresponding DS32, δ<<1. (1+mδ) is defined as a channel time unit (CU). Assume, forexample, that the generation times of the messages form a Poisson pointprocess with intensity of λ messages per unit time. λ is also calledinput rate. One sending station 24 may transmit a message in DS 32and/or a request in one CMS 34. All sending stations 24 can synchronizeon both CMS 34 and DS 32 boundaries and all sending stations 24 candetect ternary feedback information for each CMS 34 and each DS 32 fromtransmission channel 30 immediately after transmission. The assumptionof immediate feedback is unrealistic, however, the collision resolutionalgorithms can be modified to accommodate delayed feedback.

FIGS. 4a and 4b show a schematic representation of a topology whereindetecting the state of each CMS 34 can be accomplished at either commonnode 20 or at sending stations 24. It is apparent that a base node, abase station or the like, as well as any suitable passive or activeelement, can be used in lieu of common node 20. If common node 20 isused to determine the status of CMS 34, it then transmits a two bitpattern representing the ternary feedback results of each CMS 34 tosending stations 24. In such method according to one preferredembodiment of this invention, the technology required to ascertain thestatus of CMS 34 need only be built once. Such preferred method of thisinvention can also be used as a default method in systems where sendingstations 24 always transmit to a central data base rather to otherstations.

The basic principle of the tree collision resolution algorithm is toresolve one initial collision before trying to resolve another one. Inorder to decouple transmission time from arrival time, let t_(i-1)represent the instant in the transmission axis that all messages havearrived before the instant of X_(i-1) in the arrival axis and havesuccessfully resolved their conflicts, as illustrated in FIGS. 2a and2b. The interval (x_(i-1), t_(i-1)) is called the waiting interval. Theinterval (x_(i-1), x_(i)) is called the enable transmission interval(ETI), which is determined from the following formula:

    x.sub.i =x.sub.i-1 +min{W.sub.0, t.sub.i-1 -x.sub.o-1 }    Eqn. 1

where W₀ is called the default window size which is optimized byperformance requirements. Obviously, if the length of a waiting intervalis greater than the default window size, the ETI is part of the waitinginterval, as shown in FIG. 2a, otherwise the ETI is equal to the waitingwindow, as shown in FIG. 2b.

In the DQRAP according to this invention, collision resolution is basedon the ETI. Only after all messages in the current ETI have successfullyresolved their conflicts can the next ETI be initiated. If an instant tiall messages in the ETI (x_(i-1), x_(i)) have successfully resolvedtheir conflicts, the interval (t_(i-1), t_(i)) is called the contentionresolution interval (CRI) corresponding to ETI (x_(i-1), x_(i)). In theDQRAP of this invention, two distributed queues are maintained by eachsending station 24: the data transmission queue, or simply TQ, and thecollision resolution queue, or simply RQ. |TQ(t)| and |RQ(t)| representthe queue lengths of TQ and RQ at the instant t, respectively. Thephrase "transmit a request" means that a station rolls an m-sided dieand transmits a request signal in the selected minislot.

Let F_(j), where j=1,2, . . . m, denote feedback from the j-th CMS.F_(j) belongs to the set of (E,S,C), where E denotes an empty minislot,S denotes the presence of a single request signal in a minislot, and Cdenotes the presence of two or more request signals transmitted in asingle minislot.

The protocol of this invention comprises three main sets of rules: datatransmission rules (DTR), request transmission rules (RTR), and queueingdiscipline rules (QDR). A first come first scheduled (FCFS) schedulingdiscipline is used for both the TQ and the RQ but other schedulingdisciplines could be utilized. Basically the DTR, the RTR and the QDRaddress the issues: (1) who can transmit data and when; (2) who cantransmit requests and when; and (3) how does the channel feedback affectthe queues.

The following Data Transmission Rules (DTR) apply to the method of thisinvention:

(1) If (|TQ(t)|=0&&|RQ(t)|=0) then sending stations 24 with messageswhich have arrived in the current ETI transmit messages in DS 32 at time(t); and

(2) If (|TQ(t)|>0) then sending station 24 which owns the first entry inthe TQ transmits its message in DS 32 at time (t).

The following Request Transmission Rules (RTR) apply to the method ofthis invention:

(1) If (|RQ(t)|=0) then sending stations 24 with messages which havearrived in the current ETI transmit requests at time (t); and

(2) If |RQ(t)|>0 then sending stations 24 which "own" the first entry inthe RQ transmit requests at time (t).

The following Queueing Discipline Rules (QDR) apply to the method ofthis invention:

At time (t), using data slot or minislot feedback:

(1) Each sending station 24 increments |TQ(t)| for each (F_(j) (j=1, . .. m)=S);

(2) Each sending station 24 decrements |TQ(t)| by one at (t) for asuccessful message transmission commencing at (t-1);

(3) If |RQ(t)|=0 each sending station 24 increments |RQ(t)| by n where nis the number of collisions C in F_(j), where j=1, . . . m;

(4) If |RQ(t)|>0 each sending station 24 modifies |RQ(t)| by (n-1) wheren is the number of collisions, C, in F_(j),j=1, . . . m; and

(5) Sending stations 24 which transmit successful requests or collidedrequests know their position in the TQ or the RQ and adjust theirpointers or counters to the TQ or the RQ accordingly.

Using the rules presented above, the DQRAP according to this inventioncan be described by the following algorithm:

    ______________________________________                                        Set (t)=0, |TQ(t)|=0, and |RQ(t)|=0;      While (TRUE)                                                                   1) t=t+1                                                                      2) transmit data obeying the DTR;                                             3) transmit(s) request(s) obeying the RTR;                                    4) all stations modify their counters of                                       the TQ and the RQ and their pointers to                                       the TQ or the RQ following the QDR.                                         }                                                                             ______________________________________                                    

DTR(1) described above is important since it preserves the immediateaccess feature of random multiple access communications anddistinguishes the DQRAP of this invention from reservation protocols. Itis emphasized that DTR(1) may permit a collision to occur in the DS, butwithout DTR(1) the DS would otherwise be empty. DTR(1) improves thedelay characteristics of the protocol according to this invention,especially when the input rate is low.

The algorithm to resolve queueing contention in the DQRAP according toone preferred embodiment of this invention uses ternary feedback andmultiple minislots.

One preferred embodiment shown in FIG. 3 describes the operation of theDQRAP of this invention. The default window size is infinite (W_(o) =∞),for example, the ETI is equal to the waiting interval. The time axis isdivided into equal slots with length of one channel unit. Above the timeaxis the contents of the CMS and the DS are shown in FIG. 3. Below thetime axis the contents of the TQ and the RQ at each sending station 24are shown. The asterisks denote the arrival time of messages p1, p2, . .. p10. In this example two minislots are used. Assume at t=0 that boththe TQ and the RQ are empty. At t=1, p1 and p2 each transmit bothrequests and messages. At t=2 the feedback shows that the p1 and p2 datamessages have collided but their requests have not collided. p2 and p1go into the TQ and p2 data is transmitted at t=2. Meanwhile p3, arrivingin interval [1,2) transmits a request but no data since |TQ(2)|> 0. p3enters the TQ as p2 leaves. While p1 and p3 are waiting their turn totransmit data, p4, p5, and p6 transmit requests at t=3. p6's request isok and p6 enters TQ but p4 and p5 collide and thus enter the RQ. p4 andp5 collide at t=4 on their first try to resolve the collision but on thenext attempt at t=5 they succeed and enter the TQ, their orderdetermined by their relative position in the minislots. p6 transmits att=5 since the TQ operates independently of the RQ. The RQ is empty att=6 thus p7, p8 and p9, which arrived in the interval [3,5) and couldnot transmit requests or data join p10 at t=6 in making their firstattempt to transmit. p8 and p9 collide in the first minislot while p7and p10 collide in the second minislot. This determines their order inthe RQ. Such process then continues.

The diagrammatic view of FIG. 4a shows one slot time wherein thepreceding and succeeding slot times are not shown. In such preferredembodiment according to this invention, it is assumed that TQ>0, andsending station 24(2) is at the head of the queue. Sending stations24(1), 24(3) and 24(4) have requests to transmit so that they randomlyselect one CMS 34 and transmit it as a corresponding slot time. Sendingstation 24(3) selects the first CMS 34(1), as shown in FIG. 4a, and issuccessful, while sending stations 24(1) and 24(4) collide in the secondCMS 34(2). As shown in FIG. 4a, there is no transmission within CMS34(3). Following the DQRAP rules according to this invention, sendingstation 24(3) joins the transmission queue and awaits its turn totransmit, while sending stations 24(1) and 24(4) obtain exclusive use ofthe second CMS 34(2) in order to resolve their collision. The normaltransmission of data in DS 32 continues. FIG. 4a shows the status of thetransmission before reaching common node 20. FIG. 4b shows the status ofthe transmission after reaching common node 20.

As shown in FIG. 4b, DS 32 is transmitted to all sending stations24(1)-24(4), and is also transmitted to all remaining sending stations24(n). The single-crosshatched CMS 34 shown in FIG. 4b which istransmitted to all sending stations 24(1)-24(n) represents a successfulCMS 34. The double-crosshatched CMS 34 sent to each sending station24(l)-24(n) represents collided CMS 34, which is a result of thetransmission shown within the second CMS 34(2) in FIG. 4a. As shown inFIG. 4b, the gap between DS 32 and the double-crosshatched CMS 34 beingtransmitted to each sending station 24 represents the fact that therewas no transmission in the third CMS 24(3), as shown in FIG. 4a. Also asshown in FIG. 4a, station 24(2) is transmitting data and is at the headof the transmission queue.

FIGS. 4a and 4b represent one preferred logical organization of stations24(l)-24(n) and the remaining network. In practical hardwareapplications, each CMS 34 and each DS 32 can be transmitted viatransmission channel 30 which may comprise fiber, unshielded twistedpair (UTP) copper, shielded copper, coaxial cable such as that used inCATV systems, or any other suitable material. All sending stations 24can be connected to a folded cable, which is particularly useful where adual bus is employed. The network topology can be tree and branch, star,or any other suitable combination of other conventional topologies. Itis apparent that common node 20 shown in FIGS. 4a and 4b may compriseany other suitable hardware. Furthermore, a wireless station or wirelesssystem can be used and may comprise spread spectrum, and all of theconventional forms of signal transmission.

It is also apparent that FIGS. 4a and 4b can be put into practice with asatellite circuit wherein common node 20 is the satellite which acceptsincoming signals on one frequency and maps them into another frequencyfor transmission to ground sending stations 24. Such ground sendingstations 24 can transmit back on a third frequency to the satellite whenthe satellite maps the third frequency into a fourth frequency back to amain station. Depending upon the particular technology, a satellite canbe equipped with means for detecting the collisions of CMS 34 and thenfor transmitting the feedback results to ground sending stations 24along with DS 32. One advantage of such system is considerable reductionof a delay since sending stations 24 would receive the feedback afterone round trip to a satellite in approximately 250 milliseconds, in lieuof the conventional two round trips which requires approximately 500milliseconds. The CCTFB method according to this invention would besuitable to use in such a satellite circuit. In the instance ofsatellite circuits, an interleaving technique would be used to maintainthe high efficiency of the DQRAP according to this invention.

The DQRAP of this invention can be modeled as a queueing systemcomprising two subsystems, as shown in FIG. 4c: (1) a queueingcontention resolution subsystem (QCR); and (2) a data transmission (DT)subsystem. Such model can be used to evaluate the throughput of theDQRAP. DTR(1) is not considered, because it does not affect systemthroughput, as previously indicated. The servers of the QCR subsystemcan be modeled as a G/D/1 queue, the server being the data slot, theservice time being one slot per message.

Analysis of the subsystems first requires the calculation of theexpected length L_(n) of the CRI, defined as the period commencing withthe time slot containing the initial queueing contention, if any, andending with the slot in which the initial queueing contention isresolved. The variable n represents the number of sending stations 24involved in the initial queueing contention and is called themultiplicity of CRI in the contention resolution algorithm literature.For consistence, a successful transmission is defined as a conflict ofmultiplicity 1 while an empty ETI is defined as a conflict ofmultiplicity 0. With L_(n) as the expected length of CRI withmultiplicity n, L_(n) can be calculated as follows: ##EQU1##

The variable m represents the number of minislots which is chosen byperformance requirements. FIG. 5 is a table containing values of L_(n)as obtained from Eqns. 2-4 with different values of m. FIG. 5 shows thatwhen m≧3, L_(n) <n for n>1. This means a collision of multiplicity n canbe resolved in less than n slots, which is the time to transmit nmessages. Thus, the speed of contention resolution is faster than thespeed of data transmission, which is a very important aspect of thisinvention.

The DQRAP of this invention is stable if and only if both the QCRsubsystem and the DT subsystem are stable. Stability conditions of theQCR subsystem can be determined by using Markov chain theory. Themaximum stable input rate, or throughput, can be determined by thefollowing formulae: ##EQU2## FIG. 6 shows the maximum input rates andthe corresponding window sizes as a function of minislot number. FIG. 6shows that if m>2 the QCR subsystem is stable even when the input rateis greater than 1. Next consider the DT subsystem. The DT subsystem cangenerally be modeled as a G/D/1 queue. Though the QCR subsystem can bestable with the input rate greater than 1, G/D/1 is stable only when theinput rate is less than 1. Thus the DQRAP is stable when the trafficintensity is less than 1. The QCR subsystem can resolve collisionsfaster than the speed of data transmission thus guaranteeing that theQCR subsystem will not block input traffic to the whole system.

The performance of the DQRAP is determined by the QCR subsystem and theDT subsystem. The QCR subsystem does not affect data transmission, andis stable even when the traffic intensity is greater than 1 if three ormore minislots are utilized. Thus, since the QCR subsystem does notblock traffic to the whole system, the system throughput is entirelydetermined by the DT subsystem, for example, the DQRAP can achieve amaximum theoretical throughput approaching one if three or moreminislots are utilized. When the minislot overhead is included, theactual throughput, or utilization, that can be achieved is: ##EQU3##FIG. 7 shows the throughput of the DQRAP according to this invention asa function of the input rate and the number of minislots with theoverhead equal to 0.01. FIG. 8 shows the relationship of the throughputand the number of minislots. It is apparent that high actual throughputsuggests that the number of minislots selected should be as small aspossible. Fortunately, evaluation shows that with as few as threeminislots, the DQRAP achieves a maximum theoretical throughput of one.The analytical solution of delay characteristics for the DQRAP is known.Here an accurate simulation has been used to obtain the delayperformance of the DQRAP and this performance may be evaluated bycomparing it to a perfect scheduling protocol.

Simulations, based upon the algorithm previously described rather thanthe above model, have been carried out according to this invention. Thesimulations show that the DQRAP according to this invention demonstratesgood system stability, in particular all messages are guaranteed to betransmitted with a limited delay for all input rates less than or equalto 0.99. This is consistent with a system stability analysis. Theperformance bound for all random access protocols for a slottedbroadcast channel shared by an infinite number of Poisson sources isthat of a hypothetical perfect scheduling protocol, such as the M/D/1system. Thus the performance of the DQRAP of this invention is bestdemonstrated by comparison with that of the M/D/1 system. FIG. 9 showsthe ratio of the first access throughput, which is defined as the ratioof messages successfully transmitted in the first slot after theirarrival to system throughput of the DQRAP, as a function of the inputrate, using three minislots as compared to the M/D/1 system. FIG. 10contains average delay and corresponding deviation of the DQRAP, threeminislots being used, as compared with the M/D/1 system. FIG. 10 showsthat the average delay of the DQRAP is very close to the average delayof the M/D/1 system, and the maximum difference of average delaysbetween the M/D/1 system and the DQRAP of this invention is less thanthree slots when the input rate is less than 0.95. FIG. 11 showssimulated average delay and deviation of the DQRAP with a varying numberof minislots. FIG. 12 plots simulation results showing the average delayof the DQRAP along with that of an M/D/1 system. FIG. 11 shows thatincreasing the number of minislots does not impact the maximumtheoretical throughput and even though the average delay is affected bythe number of minislots it appears that for most practical purposes thenumber of minislots need not be greater than four. Finally, the DQRAPwas compared with the best known tree protocols with minislots, namely,the announced arrival random access (AARA) protocols. To achieve atheoretical throughput approaching one the announced arrival treeprotocols require an infinite number of minislots, but the DQRAP of thisinvention requires as few as three minislots. Using three minislots theannounced arrival tree protocols achieve a throughput of 0.853. TheDQRAP according to this invention provides better performance than thebest tree protocols known to date.

According to one preferred embodiment of this invention, a method forcontrolling multiple access of a transmission channel, which ispreferably a duplex channel, includes assigning a plurality of differentpatterns to sending stations 24 so that each sending station 24corresponds to a unique pattern. According to one preferred embodimentof this invention, each different pattern is represented by a differentBinomial coefficient C(n, k). Such Binomial coefficient C(n,k)represents one of a number of ways that k distinct objects can beselected from a set of n elements. The variable n is preferably in arange from 3 to 40, and for most practical applications is in a rangefrom 4 to 15.

The variable n is preferably approximately equal to two times thevariable k, such that a maximum number of different patterns can beobtained from the Binomial coefficient C(n, k). Each Binomialcoefficient can be conveniently represented by a unique binary value. Itis apparent that other patterns can be used to identify each sendingstation 24. However, by using a Binomial coefficient C(n, k), it isapparent that each unique pattern can easily be communicated throughtransmission channel 30, such as a suitable wire or fiber optic line.

Each unique pattern is transmitted from a corresponding sending station24 to transmission channel 30 by way of CMS 34. Computing means are usedto receive ternary feedback from CMS 34 and to then analyze thesummation of the different patterns within each CMS 34 to detect whethera collision exists between the different patterns within any particularCMS 34.

According to one preferred embodiment of this invention, an existingcollision is detected by using the ternary feedback to differentiatebetween an absence of the pattern or no pattern present, a presence ofonly a single pattern, or a presence of a plurality of the patternswithin any one particular CMS 34. If a collision is detected, then datato be transmitted over transmission channel 30, from each sendingstation 24, is prioritized according to the DQRAP of this invention.

In one preferred embodiment of this invention, the DQRAP functionsaccording to the DTR, RTR and QDR, as discussed above.

The DQRAP according to this invention is a medium access control methodwhich can provide performance with respect to throughput and delayapproaching that of a perfect scheduling protocol. The DQRAP is stableat all input rates of less than 1 when three or more CMS 34 areutilized. The DQRAP can be implemented by overcoming the usual problemsattendant with any conventional medium access control method. The majorchallenge is obtaining ternary feedback but it appears that this isfeasible in broadband signalling over copper, fiber, and air and withbaseband signalling on copper and fiber.

While in the foregoing specification this invention has been describedin relation to certain preferred embodiments thereof, and many detailshave been set forth for purpose of illustration it will be apparent tothose skilled in the art that the invention is susceptible to additionalembodiments and that certain of the details described herein can bevaried considerably without departing from the basic principles of thisinvention.

We claim:
 1. A method for detecting a collision in a transmissionchannel, the method comprising the steps of:(a) assigning a plurality ofdifferent patterns to a plurality of stations so that each said stationcorresponds to a unique pattern of a different Binomial coefficientC(n,k) representing One of a number of ways that k distinct objects canbe selected from a set of n elements; (b) transmitting at least one ofsaid unique patterns to the transmission channel via a control minislot;c) analyzing a summation of said unique patterns within said controlminislot to detect whether a collision exists between said differentpatterns within said control minislot; and (d) each said stationreceiving ternary feedback from said control minislot.
 2. A methodaccording to claim 1 wherein n is in a range from 3 to 40, inclusive. 3.A method according to claim 1 wherein n is in a range from 4 to 15,inclusive.
 4. A method according to claim 1 wherein n=2k.
 5. A methodaccording to claim 1 wherein detecting whether said collision existsfurther comprises using said ternary feedback to differentiate betweenan absence of said pattern, a presence of a single said pattern and apresence of a plurality of said patterns within said control minislot.6. A method according to claim 1 wherein each said unique signal patternis represented by a unique binary value.
 7. A method according to claim1 wherein upon detection of said collision, data to be transmitted fromeach said station is prioritized according to a protocol.
 8. A methodaccording to claim 7 wherein said protocol functions according to datatransmission rules (DTR), request transmission rules (RTR), and queueingdiscipline rules (QDR).
 9. A method according to claim 8 whereinaccording to said data transmission rules (DTR), if the length of a datatransmission queue (TQ) at a time (t) is equal to zero and the length ofa collision resolution queue (RQ) at said time (t) is equal to zero,then said [sending]stations having a data message arriving in a currentenable transmission interval (ETI) transmit said data message in a dataslot of said transmission channel at said time (t).
 10. A methodaccording to claim 8 wherein according to said data transmission rules(DTR), if a data transmission queue (TQ) at a time (t) has a lengthgreater than zero, then the one of said stations having the first entryof a message in said data transmission queue (TQ) transmits said messagein a data slot of said transmission channel at said time (t).
 11. Amethod according to claim 8 wherein according to said requesttransmission rules (RTR), if the length of a collision resolution queue(RQ) at a time (t) is equal to zero, then said stations having a messagearriving in a current enable transmission interval (ETI) each transmitsa request at said time (t).
 12. A method according to claim 8 whereinaccording to said request transmission rules (RTR), if a collisionresolution queue (RQ) at a time (t) has a length greater than zero, thensaid stations having a first entry of a message in said collisionresolution queue (RQ) transmit a request at said time (t).
 13. A methodaccording to claim 8 wherein according to said queueing discipline rules(QDR), each said station increments the length of a data transmissionqueue (TQ) by one for each ternary feedback signal of said controlminislot indicating a presence of a single said different pattern withinsaid minislot.
 14. A method according to claim 8 wherein according tosaid queueing discipline rules (QDR), each said station decrements thelength of a data transmission queue (TQ) by one for a messagetransmission commencing before a time (t).
 15. A method according toclaim 8 wherein according to said queueing discipline rules (QDR), ifthe length of a collision request queue (RQ) at a time (t) is equal tozero, then each said station increments the length of said collisionrequest queue (RQ) by a number n, where said number n is equal to anumber of collisions detected within said control minislot.
 16. A methodaccording to claim 8 wherein according to said queueing discipline rules(QDR), if a collision request queue (RQ) at a time (t) has a lengthgreater than zero, then each said station increments the length of saidcollision request queue (RQ) by a number n-1, where said number n-1 isequal to one less than a number of collisions detected within saidcontrol minislot.
 17. A method according to claim 8 wherein according tosaid queueing discipline rules (QDR), each said station learns acorresponding position of said station in one of a data transmissionqueue (TQ) and a collision request queue (RQ) and accordingly adjusts apointer to one of said data transmission queue (TQ) and said collisionrequest queue (RQ).